Gas driven pumps for pumping fluids such as beverage syrups are well known. Such gas driven pumps are commonly utilized in carbonated beverage fountains wherein the pump is powered via carbon dioxide so as to effect dispensing of the syrup and/or carbonated water comprising a beverage.
One example of such a gas driven pump is that disclosed in U.S. Pat. No. 4,540,349 issued on Sep. 10, 1985 to Du, the contents of which are hereby incorporated by reference. In that contemporary gas driven pump, two opposed pistons are mounted upon a common piston shaft for reciprocal movement thereof within a housing. Cavities complimentary to the pistons are alternately vented and pressurized to intake the pumped product into the pair of cylinders and to drive the pistons so as to pump the product.
A spool valve stem extends into the cavity being vented so that the piston performing an intake stroke moves the valve stem into the other cavity so as to effect alternating motion of the two pistons. A pair of axial passages and corresponding side openings in the valve stem provide fluid flow paths for venting and pressurizing the cavities. A valve body biased toward the cavity being vented includes a vent passage for alternately venting the cavities through side inlets. While one cavity vents, pressurized fluid flows into the other cavity through the corresponding side inlet and axial passage. The valve body moves with the valve stem until the biasing spring is in an unstable over/center position, where the bias reverses to urge the valve body toward the other cavity.
Although such contemporary gas driven pumps have proven generally suitable for their intended purposes, such gas driven pumps suffer from inherent deficiencies which detract from their overall performance and utility. More particularly, such contemporary gas driven pumps are subject to stalling wherein the valve mechanism for effecting alternating pressurization and venting of the cylinders sticks, and consequently does not continue the cyclic pressurization/venting process.
The occurrence of such stalling is often exacerbated by the use of a pressurized gas, such as carbon dioxide, wherein expansion of the pressurized gas into the cylinders and/or body of the air driven pump results in substantially reduced temperatures at the inlet orifices of the gas driven pump. Such reduced temperatures are frequently accompanied by frost buildup at the inlet orifices which impedes movement of the valve mechanism, thereby resulting in stalling of the gas driven pump.
A further deficiency of such contemporary gas driven pumps is their inability to sense depletion of the pump product and automatically shut off when such depletion is sensed. Rather, such contemporary gas driven pumps continue attempting to pump the pumped product even after the supply of pumped product has been deleted.
Prolonged operation of a gas driven pump subsequent to depletion of the pumped product may result in excessive wear thereto. The pumped product provides a degree of lubrication and cooling to the gas driven pump. As such, it is undesirable to operate gas driven pumps for a prolonged period of time subsequent to deletion of the pumped product.
As such, it is beneficial to provide a gas driven pump which is not substantially subject to stalling and which more particularly inhibits the buildup of frost so as to prevent sticking of the valve mechanism. It is also beneficial to provide a gas driven pump which automatically shuts off when depletion of the pumped product is sensed.